An autistic man who convinced relatives his mother was alive years after she vanished has been sentenced to 20 years in prison for her murder.

Mount Hawthorn woman Ah Bee Mack,54, a Chinese-Australian known to friends and family as Pauline, had a reclusive lifestyle in her final years.

Brent Donald Mack, one of her two sons, reconnected with Mrs Mack during that time, moving back into the family home the year of her disappearance.

He was convicted of her murder by a judge alone in November 2012.

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At trial Mack, 27, maintained he had not killed his mother and had instead found her dying of self inflicted wounds and promised to keep her death a secret.

Although she was last seen alive in late 2008, Mrs Mack was only reported missing in 2010, because her son had spun an elaborate web of lives to convince relatives she was still alive and living in hiding.

Justice John McKechnie found Mack had killed his mother to benefit financially from the lucrative real estate assets she had inherited only months before her death from her late husband.

After his mother's death, Mack moved almost immediately to start siphoning hundreds of thousands of dollars out of her accounts.

Mack kept his head bowed and eyes closed through his sentencing in the Supreme Court of WA on Friday.

Psychological and psychiatric assessments of Mack had ruled out his autism as a factor in the murder.